Method of electrophotographically manufacturing a television screen using hygroscopic material

ABSTRACT

A method of electrophotographically manufacturing a display screen of a colour television display tube. For enhancing the contrast, a light-absorbing layer is provided between the phosphor patterns. In order to prevent pollution of the previously provided phosphor patterns with the phosphor of phosphor patterns which are provided afterwards, the phosphor particles are previously provided with a layer of a hygroscopic material. The hygroscopic material is wetted and hence made conductive between the provision of two phosphor patterns. The hygroscopic material is dried and hence made non-conductive between the provision of the last phosphor pattern and the provision of the light-absorbing layer.

Strik et a1.

U.S. Philips Corporation, New York, NY.

Filed: Feb. 26, 1973 Appl. No.: 335,942

Assignee:

Foreign Application Priority Data Mar. 4, 1972 Netherlands 7202907 U.S.Cl 96/1 R, 96/361, 117/335 CM. 117/335 CP Int. Cl G03g 13/22, C030 3/28Field 01' Search 96/1 R, 1.2, 36.1; 117/33.5 CM, 33.5 CF

References Cited UNITED STATES PATENTS 6/1964 Hays et a1. 96/1 R X 1Dec. 24, 1974 3,365,292 1/1968 Fiore et a], 96/361 3,475,169 10/1969Lange 96/1 R 3,489,556 1/1970 Drozd 96/1 R 3,489,557 1/1970 Lange et al.117/335 CM 3,615,459 10/1971 Kaplan 117/3315 CM 3,615,462 10/1971 Szeghoet al 96/361 3,632,339 1/1972 Khan 96/361 3,745,398 7/1973 Oikawa96/36.] X

Primary Examiner-Roland E. Martin, Jr. Attorney, Agent, or FirmFrank R.Trifari [57] ABSTRACT A method of electrophotographically manufacturinga display screen of a colour television display tube. For enhancing thecontrast, a light-absorbing layer is provided between the phosphorpatterns. In order to prevent pollution of the previously providedphosphor patterns with the phosphor of phosphor patterns which areprovided afterwards, the phosphor particles are previously provided witha layer of a hygroscopic material. The hygroscopic material is wettedand hence made conductive between the provision of two phosphorpatterns. The hygroscopic material is dried and hence madenon-conductive between the provision of the last phosphor pattern andthe provision of the light-absorbing layer.

1 Claim, 1 Drawing Figure METHOD OF ELECTROPI-IOTOGRAPHICALLYMANUFACTURING A TELEVISION SCREEN USING IIYGROSCOPIC MATERIAL Theinvention relates to a method of electrophotographically manufacturing adisplay screen of a colour television display tube comprising the stepsof: providing a conductive layer which can be fired on a face panel ofthe tube and providing a photoconductive layer which can be fired on theconductive layer; providing a first pattern of phosphor regions byuniformly electrically charging the photoconductive layer, forming alatent charge image which comprises charge in the regions correspondingto the said first pattern by exposing the photoconductive layer to lightthrdugh a mask, and depositing phosphor particles charged with oppositepolarity from an insulating developing liquid on the said charge image;providing in an analogous manner at least one subsequent pattern ofphosphor regions; providing a light absorbing layer between the phosphorregions by uniformly electrically charging the photoconductive layerwith the phosphor regions, uniformly exposing to light thephotoconductive layer with the phosphor regions, and depositinglightabsorbing particles charged with the same polarity from aninsulating developing liquid between the charged phosphor regions;firing the face panel for removing the conductive layer and thephotoconductive layer. The invention also relates to a colour televisiondisplay tube manufactured by said method.

Such a method is known from the US. Pat. No. 3,475,l69. This patentdescribes a few variations of the electrophotographic method. In a firstvariation which is termed direct imaging, the charged particles aredeposited on the charged regions of the charge image because the chargedparticles have the opposite polarity of the charge image. In a secondvariation which is termed reverse imaging, the charged particles aredeposited between the charged regions of the charge image because thecharged particles have the same polarity as the charge image. Moreover,two exposure methods are possible. In the first method, a positive lightpattern is formed of the apertures in a colour selection electrode. in asecond method a negative light pattern is formed of the apertures in acolour selection electrode, that is to say that each aperture in thecolour selection electrode is reproduced by a shadow region on thephotoconductive layer.

In a given type of colour television display tube the phosphor regionson the display screen are smaller or narrower than the electron spots.This is of great importance since in that case a much smaller part ofthe surface of the display screen is occupied by phosphor regions andthe intermediate space can be filled with a light-absorbing material.This has for its result that the colour television display tube canprovide a much greater contrast because ambient light reflected by thescreen is minimized. The tolerance which is necessasry between thedimensions of the phosphor regions and the dimensions of the electronspots is produced in that the electron spots overlap the phosphorregions.

A colour television display tube of the shadow mask type in which thephosphor regions are smaller than the electron spots and are separatedby regions having a light-absorbing material is known from the US. Pat.No. 3,l46,368. This patent also describes a few methods of manufacturingsuch a tube. These methods come down to the fact that by carefullydosing the exposure to light, phosphor regions are manufactured on thescreen which are smaller than the apertures in the colour selectionelectrode. This method is very difficult to perform in practice, whilethe achieved reduction farther is also not sufficient. Another knownmethod starts from a colour selection electrode having small provisionalapertures. After using the colour selection electrode for the exposure,said apertures are made larger by after-etching or by removing a layerwhich was provided thereon temporarily. Of course this method is verycumbersome, which is a decisive drawback in practice.

In the US. Pat. No. 3,6 I 5,459 a method is described with whichphosphor regions which are smaller than the apertures in the colourselection electrode can be manufactured on the display screen in asimple and noncritical manner. In this method, each aperture in thecolour selection electrode is represented during the exposure by ashadow region on the photoconductive layer. This exposure method is verysuitable for use in combination with the electrophotographic provisionof the phosphor regions and the light-absorbing layer. This is actuallydue to the free choice between the direct and reverse imaging which onehas in a electrophotographic method. The exposure method described inthe US. Pat. No. 3,615,459 combined with the electrophotographic methoddescribed in the US. Pat. No. 3,475,169, however, shows the drawbackthat previously provided patterns of phosphor regions are slightlypolluted by the phosphor of patterns provided afterwards.

It is an object of the invention to prevent pollution of previouslyprovided patterns by the phosphor and/or the light-absorbing material ofpatterns provided afterwards. Another object of the invention is toprovide a method of manufacturing a colour television display tube whichis capable of displaying strongly saturated colours.

According to the invention, a method of the type described in the firstparagraph is characterized in that, prior to mixing the phospor phosphorwith the developing liquid, the phosphor particles for at least one ofthe patterns are provided with a layer of a hygroscopic material whichis electrically conductive in the moist condition, that between theprovision of two patterns of phosphor regions the hygroscopic materialon the face panel is brought into a moist condition and that between theprovision of the last pattern of phosphor regions and the provision ofthe light-absorbing layer the hygroscopic material on the face panel isbrought into a dry condition.

The invention is based on investigations which have proved that thephosphor regions of already provided patterns have insufficientelectrical conductivity to be discharged during the subsequent exposurewhich is necessary to provide a subsequent pattern. For that purpose itwould be necessary for the phosphor parti cles to be made slightlyconductive. The provision of a conductive layer on the phosphorparticles as such, however, serves no purpose, for in that case thephosphor regions would also lose their charge during the last uniformexposure for providing the light-absorbing layer. So it must be possiblefor the conductive phosphor particles to be made non-conductive again.According to the invention, a hygroscopic material is used for thatpurpose which in itself is non-conductive but which is made conductiveby the absorption of moisture and is afterwards made non-conductive bydrying.

It is to be noted that the influence of possible conductivity of thephosphor regions is stated in column 12 of the already mentioned U.S.Pat. No. 3,475,169. Here, however, it is described that the patterns ofphosphor regions are made conductive afterwards by means of an agentwhich also serves as a fixative. According to said patent this wouldhave for its result that during the electric charge of thephotoconductive layer the charge of the phosphor regions leaks away, asa result of which the light-absorbing layer could by provided by amethod of direct imaging and without exposure. Apart from the fact thatsaid method does not prove satisfactory in practice, no solution isgiven in said passage for the problem of the pollution of the phosphorpatterns.

The invention will be described in greater detail with reference to thefollowing example and by means of the accompanying drawing which shows apart of a display screen of a colour television display tube during theperformance of the method according to the invention.

The drawing shows a face plate 1 of a colour television display tube ofthe shadow mask type. An organic conductive layer 2 consisting ofpolyvinyl piperidinium chloride is provided on the face plate 1. Anorganic photoconductive layer 3 consisting of polyvinyl carbazol isprovided on the conductive layer 2. A colour selection electrode 4having apertures 5 is arranged immediately in front of the face plate Iat exactly the same distance as afterwards in the operating tube. Alight source 6 of comparatively large dimensions throws light throughthe apertures 5. ln practice, a correction lens for accurate matching ofthe place of the light source to the deflection point of the electronbeam in the operating tube is arranged between the light source 6 andthe shadow mask 4. The light spots 7 on the conductive layer 3 show ahalf-shadow 8. As a result of this, the light spots are considerablylarger than the apertures 5. Prior to the exposure to light, thephotoconductive layer 3 is provided with a negative surface charge. Thisis carried out by exposing the layer 3 to a corona discharge of anelectrode which is at a high potential relative to the conductive layer2. This is a known method which is also described in the U.S. Pat. No.3,475,169. In the places illuminated by the light source thephotoconductive layer 3 becomes conductive as a result of which thenegative surface charge leaks away to the conductive layer 2. The lightsource 6 is first arranged in the deflection point of the electron beamwhich in the operating tube is to impinge upon the discrete regions withgreen phosphor. The light source is then arranged in the deflectionpoint for blue. After this, only parts of the photoconductive layerwhich have not been exposed to light still contain negative electriccharge. After removing the shadow mask 4, a suspension which comprisespositively charged red luminescing phosphor particles in an insulatingliquid is sprayed against the photoconductive layer. The said liquidconsists of branched hydrocarbons, for example. a mixture of octane andnonane. Before the phosphor particles were suspended with the insulatingliquid, they have been covered with a thin layer of a hygroscopicmaterial for which, for example, a mixture of polyvinyl alcohol, cholinechloride and glycerin is very suitable. The red phosphor adheres to theoppositely charged unexposed places which form a pattern of dots havingdiameters smaller than the diameters of the apertures 5.

It is to be noted that in direct imaging, a single exposure from thedeflection point for the colour to be provided will also be sufficientwhen a special form of the light source is used as is described in theU.S. Pat. No. 3,152,900. With such an annular light source it is alsopossible to perform the exposure in such manner that the unexposed partsform a pattern of dots having diameters smaller than the diameters ofthe apertures 5.

After drying the provided layer of red phosphor dots, the display screenis exposed to moist air for some time. As a result of this thehygroscopic material with which the phosphor grains are covered absorbssome water and the phosphor dots become conductive.

The photoconductive layer 3 with the conductive phosphor dots is thenagain provided with a negative surface charge and exposed to light forproviding the next pattern of phosphor dots. Since the red phosphor dotsare conductive, the surface charge thereof can leak away via the exposedparts of the photoconductive layer 3. The next patterns of phosphor dotsare provided in an entirely analogous manner.

The display screen provided with all the patterns of phosphor dots,usually red, blue and green, is then heated in dry air as a result ofwhich the water is expelled from the hygroscopic material around thephosphor grains. The photoconductive layer 3 is then provided with apositive surface charge. After uniform exposure of the photoconductivelayer 3, said charge leaks away everywhere with the exception of thephosphor dots which are now non-conductive. The space between thephosphor dots is then filled with a lightabsorbing material. For thispurpose, a suspension is sprayed against the photoconductive layer 3which consists of positively charged particles of a black pigmentationagent, for example graphite or a black metal oxide, in an insulatingliquid. The light-absorbing material is repelled by the charge of thesame sign of the phosphor dots and adheres to the remaining surfacebetween the phosphor patterns.

The organic layers 2 and 3 are then removed by firing after which thephosphors and the light-absorbing substance immediately adhere to theglass of the display screen 1.

It will be obvious that the invention is not only suitable formanufacturing a colour television display tube of the shadow mask typebut that it may also be used in manufacturing other types of displaytubes. The problem of pollution of previously provided patterns with thephosphorof patterns provided afterwards actually always occurs in thosecases where the direct imaging as stated in the U.S. Pat. No. 3,475,169is used for providing the phosphor patterns and/or subsequently alight-absorbing layer between the phosphor patterns is provided with thereverse imaging.

What is claimed is:

l. A method of electrophotographically manufacturing a display screen ofa color television display tube comprising a shadow mask and a facepanel, said method comprising the steps of:

a. providing said face panel comprising a fireable conductive layerdisposed thereon and a fireable photoconductive layer disposed on saidconductive layer;

b. providing first phosphor particles comprising an outer layer of ahygroscopic material which is electrically conductive in a moistcondition;

face panel by uniformly electrostatically charging said photoconductivelayer and selectively discharging said photoconductive layer by exposingf. rendering said hygroscopic material on said face panel in a drycondition;

g. following step (f) providing a light-absorbing layer between saidphosphor regions by uniformly elecsaid photoconductive layer through ashadow 5 trostatically charging said photoconductive layer mask, therebyforming a latent charge image which with said phosphor regions,selectively discharging comprises charges in the regions correspondingto regions of said photoconductive layer and said dethe said firstpattern and then depositing on the posited phosphor regions by uniformlyexposing said charge image and from an insulating developsaidphotoconductive layer with the said phosphor ing liquid, first phosphorparticles charged with op- 10 regions. and depositing between thecharged phosposite polarity to said latent charge image;

phor regions and from an insulating developing liqd. rendering saidhygroscopic material of said first uid, light-absorbing particlescharged with the phosphor particles in a moist condition; same polarityas said photoconductive layer; and e. then repeating step (c) above withsecond phosh. firing the face panel to remove said conductive phorparticles to provide second phosphor regions [5 layer and saidphotoconductive layer at said face panel;

1. A METHOD OF ELECTROPHOTOGRAPHICALLY MANUFACTURING A DISPLAY SCREEN OFA COLOR TELEVISION DISPLAY TUBE COMPRISING A SHADOW MASK AND A FACEPANEL, SAID METHOD COMPRISING THE STEP OF: A. PROVIDING SAID FACE PANELCOMPRISING A FIREABLE CONDUCTIVE LAYER DISPOSED THEREON AND A FIREABLEPHOTOCONDUCTIVE LAYER DISPOSED ON SAID CONDUCTIVE LAYER; B. PROVIDINGFIRST PHOSPHOR PARTICLES COMPRISING AN OUTER LAYER OF A HYGROSCOPICMATERIAL WHICH IS ELECTRICALLY CONDUCTIE IN A MOIST CONDITION; C.PROVIDING A FIRST PATTERN OF PHOSPHOR REGIONS AT SAID FACE PANEL BYUNIFORMLY ELECTROSTATICALLY CHARGING SAID PHOTOCONDUCTIVE LAYER ANDSELECTIVELY DISCHARGING SAID PHOTOCONDUCTIVE LAYER BY EXPOSING SAIDPHOTOCONDUCTIVE LAYER THROUGH A SHODOW MASK, THEREBY FORMING A LATENTCHARGE IMAGE WHICH COMPRISES CHARGES IN THE REGIONS CORRESPONDING TO THESAID FIRST PATTERN AND THEN DEPOSITING ON THE SAID CHARGE IMAGE AND FROMAN INSULATING DEVELOPING LIQUID, FIRST PHOSPHOR PARTICLES CHARGED WITHOPPOSITE SPONDING TO THE SAIDLATENT CHARGE IMAGE; D. RENDERING SAIDHYGROSCOPIC MATERIAL OF SAID FIRST PHOSPHOR PARTICLES IN A MOISTCONDITION; E. THEN REPEATING STEP (C) ABOVE WITH SECOND PHOSPHORPARTICLES TO PROVIDE SECOND PHOSPHOR REGIONS AT SAID FACE PANEL; F.RENDERING SAID HYGROSCOPIC MATERIAL ON SAID FACE PANEL IN A DRYCONDITION; G. FOLLOWING STEP (F) PROVIDING A LIGHT-ABSORBING LSAYERBETWEEN SAID PHOSPHOR REGIONS BY UNIFORMLY ELECTROSTATICALLY CHARGINGSAID PHOTOCONDUCTIVE LAYER WITH SAID PHOSPHOR REGIONS, SELECTIVELYDISCHARGING REGIONS OF SAID PHOTOCONDUCTIVE LAYER AND SAID DEPOSITEDPHOSPHOR REGIONS BY UNIFORMLY EXPOSING SAID PHOTOCONDUCTIVE LAYER WITHTHE SAID PHOSPHOR REGIONS, AND DEPOSING BETWEEN THE CHARGED PHOSPHORREGIONS AND FROM AN INSULATING DEVELOPING LIQUID, LIGHT-ABSORBINGPARTICLES CHARGED WITH THE SAME POLARITY AS SAID PHOTOCONDUCTIVE LAYER,AND H. FIRING THE FACE PANEL TO REMOVE SAID CONDUCTIVE LAYER AND SAIDPHOTOCONDUCTIVE LAYER.